1. Field of the application
The present invention relates to foundry practice, and more particularly to induction melting furnaces.
The invention is broadly applicable to adaptation in melting of cast iron and non-ferrous alloys.
Induction furnaces have found their greatest use in foundry practice.
2. Description of the prior art
The furnaces in question normally comprise a water-cooled cooper inductor accommodating a crucible formed by way of packing, drying and sintering powder-like oxidized materials. The sintered materials form a refractory lining made in the form of a tapered cylinder with a bottom wherein a metal is melted under the effect of alternating magnetic field induced by the inductor.
The power developed in the metal during induction heating depends on the amount of metal in the furnace and upon its physical properties (specific electrical resistance, relative permeability), as well as upon the frequency and strength of electromagnetic field.
In the course of metal melting, the refractory lining, subjected to thermochemical and mechanical effects of molten metal, is destroyed and the molten metal, getting onto the water-cooled inductor, causes the latter to melt down, while the water, coming into contact with the molten metal, causes vigorous vapor generation accompanied by metal splashing.
To prevent complete destruction of the furnace refractory lining, protecting means are used to signal the degree of the refractory lining destruction (wear-out) over its thickness. However, the protecting means in question are not effective enough, since they fail to provide full picture of the refractory lining destruction. This problem gave rise to extensive research work carried out in many countries with the purpose to develop effective signal systems intended to indicate the actual state of refractory linings, as well as means for protecting the furnace inductor.
The necessity to tackle the problem in hand is governed not only by technological and economic grounds, but by safety measures as well to be taken during metal splashing.
Therefore, the solution to the above-stated problems is to be sought after as follows:
by way of installing intermediate the crucible and inductor an electrode assembly intended to produce a signal through appropriate instruments indicating the degree of the refractory lining wear;
by way of interposing a heat-resistant screen between the inductor and refractory lining, intended to prevent molten metal from falling onto the inductor.
There are known induction furnaces which are provided (with the purpose of preventing molten metal from getting onto an inductor) with a copper cylinder mounted intermediate an inductor and a crucible, split along its generatrix and equal in height to the crucible (cf. U.S.S.R. Inventor's Certificate No. 405,008). It should be observed, however, that the water-cooled copper cylinder is complicated to manufacture; it is suitable only for a certain type of furnaces; it fails to prevent failure in operation when molten metal falls onto the water-cooled cylinder and causes burning-through thereof; the cylinder is inconvinient for mounting in the furnace and reduces the working space thereof.
The water-cooled cylinder protects the inductor from destruction, being, on the other hand, the source of failure due to molten metal penetration thereon.
U.S.S.R. Inventor's Certificate No. 290,919 discloses a control means having a metallized refractory glass fabric interposed between a refractory lining and the inductor insulation, operating to detect and signal the degree of wear of the induction furnace crucible. Although this means improves operating reliability of the induction furnace, it, however, fails to protect the inductor from destruction when molten metal penetrates thereto.
U.S.S.R. Inventor's Certificate No. 35,602 describes an alarm means which is used to control the state of the crucible refractory lining. This alarm means comprises two sets of open electrodes having opposite polarity and uniformly interposed between the layers of the crucible refractory lining over the entire circumferance and height thereof. The electrodes are alternately spaced apart from one another within the distance of 5 to 8 mm.
The disadvantage of the invention referred to above lies in that it offers control only over the state of the crucible refractory lining while failing to provide protection to the inductor from molten metal passed in the interspace between the electrodes.
The difficulty encountered in mounting and connecting electrodes as well as in packing the furnace with the electrodes having been arranged therein, renders the means under consideration impracticable. The interposition of the electrodes between the layers of the refractory lining reduces the furnace working capacity. Also, the control means is unsuitable to various types of furnaces.
FRG Patent No. 1,220,085 describes an induction furnace in which there is interposed between an induction coil and a packed crucible an insert made from a heat-resistnat material and tapered from the inductor side so that the thickness of its walls gradually increases from the bottom upwards. In the furnace of the patent referred to above the intermediary insert is used to prevent metal penetration (in case it is closed throughout the inductor perimeter from the top downwards). However, this insert fails to signal the penetration of metal through the refractory lining of the furnace bottom; it is complicated and expensive to manufacture, being unsuitable to various types of furnaces and reducing the furnace working space.
F.R.G. Patent No. 1,208,451 describes a means intended to signal destruction of the induction furnace melting crucible, comprising two substantially cylindrical electrodes insulated from each other and connected to a power source through a control instrument. The electrodes are basically coaxial metallic cylinders insulated from each other by means of a packing and fixed on the crucible exterior wall intermediate the crucible and the induction coil, the outer cylinder being fitted with more openings and insulated from the induction coil.
Though the coil means of the patent referred to above is capable of preventing metal penetration (in case it is closed through-out the inductor perimeter from the top downwards), it fails to signal metal penetration through the furnace bottom, affords no protection to the latter, it is complicated and expensive to manufacture, unsuitable for various types of furnaces, and reduces the furnace working space.
F.R.G. Patent No. 1,220,086 discloses a device intended to signal the onset of destruction and protecting a melting crucible from complete destruction, which is basically an electrode interposed in the layer of an insulation material between the crucible wall and an induction coil. The electrode is connected through a power source and an indicator to the inner surface of the crucible, being as well formed with longitudinal slots. At the sections disposed at the edges of the induction coils, the electrode is divided into longitudinal sections over its entire perimeter.
The signal means of the patent referred to above suffers from a number of disadvantages, i.e. it fails to afford protection to the inductor from metal penetration; it is expensive to manufacture and is unsuitable for various types of furnaces; also, it affords no protection to, and fails to signal the destruction of, the refractory lining of the furnace bottom.
From the description of the above-mentioned signal device it follows that it is not able to simultaneously afford protection to the furnace inductor from metal penetration and signal the degree of the refractory lining wear.
Therefore, the primary disadvantage of the prior-art signal device lies in that there still remains the danger of emergency even in the event of signal sent to indicate metal penetration to the electrode through the refractory lining.
Therefore, in cases like these the metal melting process is instantaneously terminated until complete cooling of metal in the crucible, which makes it difficult to remove the cooled metal from the furnace and prolongs the time period required to render the furnace operative again.